Periodic parameter estimation for visual-inertial tracking systems

ABSTRACT

A method for calibrating a visual-inertial tracking system is described. A device operates the visual-inertial tracking system without receiving a tracking request from a virtual object display application. In response to operating the visual-inertial tracking system, the device accesses sensor data from sensors at the device. The device identifies, based on the sensor data, a first calibration parameter value of the visual-inertial tracking system and stores the first calibration parameter value. The system detects a tracking request from the virtual object display application. In response to the tracking request, the system accesses the first calibration parameter value and determines a second calibration parameter value from the first calibration parameter value.

CROSS REFERENCE

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 63/131,981, filed Dec. 30, 2020, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The subject matter disclosed herein generally relates to a visualtracking system. Specifically, the present disclosure addresses systemsand methods for calibrating visual-inertial tracking systems.

BACKGROUND

An augmented reality (AR) device enables a user to observe a scene whilesimultaneously seeing relevant virtual content that may be aligned toitems, images, objects, or environments in the field of view of thedevice. A virtual reality (VR) device provides a more immersiveexperience than an AR device. The VR device blocks out the field of viewof the user with virtual content that is displayed based on a positionand orientation of the VR device.

Both AR and VR devices rely on motion tracking systems that track a pose(e.g., orientation, position, location) of the device. A motion trackingsystem is typically factory calibrated (based on predefined/knownrelative positions between the cameras and other sensors) to accuratelydisplay the virtual content at a desired location relative to itsenvironment. However, factory calibration parameters can drift over timeas the user wears the AR/VR device due to mechanical stress andtemperature changes in the AR/VR device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

To easily identify the discussion of any particular element or act, themost significant digit or digits in a reference number refer to thefigure number in which that element is first introduced.

FIG. 1 is a block diagram illustrating an environment for operating anAR/VR display device in accordance with one example embodiment.

FIG. 2 is a block diagram illustrating an AR/VR display device inaccordance with one example embodiment.

FIG. 3 is a block diagram illustrating a visual inertial tracking systemin accordance with one example embodiment.

FIG. 4 is a block diagram illustrating a tracking calibration module inaccordance with one example embodiment.

FIG. 5 is a flow diagram illustrating a parameter estimation operationin accordance with one example embodiment.

FIG. 6 is a block diagram illustrating a process in accordance with oneexample embodiment.

FIG. 7 is a flow diagram illustrating a method for storing the latestestimated parameter value in accordance with one example embodiment.

FIG. 8 is a flow diagram illustrating a method for calibrating a visualinertial tracking system in accordance with one example embodiment.

FIG. 9 is a flow diagram illustrating a method for triggering the visualinternal tracking system in accordance with one example embodiment.

FIG. 10 is a flow diagram illustrating a method for calibrating avisual-inertial tracking system in accordance with one exampleembodiment.

FIG. 11 is block diagram showing a software architecture within whichthe present disclosure may be implemented, according to an exampleembodiment.

FIG. 12 is a diagrammatic representation of a machine in the form of acomputer system within which a set of instructions may be executed forcausing the machine to perform any one or more of the methodologiesdiscussed herein, according to one example embodiment.

DETAILED DESCRIPTION

The description that follows describes systems, methods, techniques,instruction sequences, and computing machine program products thatillustrate example embodiments of the present subject matter. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide an understanding of variousembodiments of the present subject matter. It will be evident, however,to those skilled in the art, that embodiments of the present subjectmatter may be practiced without some or other of these specific details.Examples merely typify possible variations. Unless explicitly statedotherwise, structures (e.g., structural Components, such as modules) areoptional and may be combined or subdivided, and operations (e.g., in aprocedure, algorithm, or other function) may vary in sequence or becombined or subdivided.

Both AR and VR applications allow a user to access information, such asin the form of virtual content rendered in a display of an AR/VR displaydevice (also referred to as a display device). The rendering of thevirtual content may be based on a position of the display devicerelative to a physical object or relative to a frame of reference(external to the display device) so that the virtual content correctlyappears in the display. For AR, the virtual content appears aligned witha physical object as perceived by the user and a camera of the ARdisplay device. The virtual content appears to be attached to thephysical world (e.g., a physical object of interest). In order to dothis, the AR display device detects the physical object and tracks apose of the AR display device relative to a position of the physicalobject. A pose identifies a position and orientation of the displaydevice relative to a frame of reference or relative to another object.For VR, the virtual object appears at a location based on the pose ofthe VR display device. The virtual content is therefore refreshed basedon the latest pose of the device.

A tracking system (also referred to as a visual-inertial trackingsystem) at the display device determines the latest position or pose ofthe display device. An example of a tracking system includes avisual-inertial tracking system (also referred to as visual odometrysystem) that relies on data acquired from multiple sensors (e.g., depthcameras, inertial sensors). The tracking system calibrates the sensorsto accurately determine the pose of the display device. The calibratedparameters include extrinsic parameters (e.g., relative orientations andpositions between sensors), and intrinsic parameters (e.g., internalcamera or lens parameters).

Although the tracking system of the display device is factorycalibrated, these (extrinsic/intrinsic) parameters may change over time(e.g., due to mechanical stress, temperature changes). The trackingsystem mitigates the changes by gradually updating the parameter valuesduring runtime of the AR/VR application. However, the more the parametervalues have diverged from the factory calibration, the longer it takesfor the tracking system to “catch up” and obtain an accurate newestimate of the parameter values (e.g., the convergence time becomeslonger). Keeping this convergence time short is important, because aslong as the estimates are inaccurate, tracking performance is negativelyimpacted. Furthermore, the parameter values may be used by componentsother than the tracking system. For applications that rely on thesecomponents (e.g., VR/AR systems), it is important that the trackingsystem operates accurately right from the start (e.g., when the AR/VRapplication starts or is online).

The present application describes a method for reducing the convergencetime for online parameter estimation at the start of a tracking system.Instead of just operating the tracking system whenever requested by theAR/VR application, the tracking system is periodically started and runfor a short time (e.g., just until a new parameter estimation isobtained). This new parameter value estimation is then stored and laterreused as a more up-to-date value (e.g., a new starting value forparameter estimation). Therefore, the last up-to-date parameter value isgenerally a more accurate starting point for parameter estimation whenthe tracking system is started the next time. Starting from the lastup-to-date parameter value leads to a reduced convergence time andhigher tracking accuracy immediately after the start of the trackingsystem.

In one example embodiment, the present application describes a methodfor calibrating a visual-inertial tracking system comprising: operating,at a device, the visual-inertial tracking system without receiving atracking request from a virtual object display application; in responseto operating the visual-inertial tracking system, accessing sensor datafrom a plurality of sensors of the device; identifying, based on thesensor data, a first calibration parameter value of the visual-inertialtracking system; storing the first calibration parameter value;detecting the tracking request from the virtual object displayapplication to the visual-inertial tracking system; and in response todetecting the tracking request, accessing the first calibrationparameter value and determining a second calibration parameter valuefrom the first calibration parameter value.

As a result, one or more of the methodologies described hereinfacilitate solving the technical problem of power consumption saving andefficient calibration by periodically operating the tracking system todetermine a parameter value, storing the latest parameter value, andusing the last parameter value as a starting point the next time thetracking system is requested by an application. The presently describedmethod provides an improvement to an operation of the functioning of acomputer by providing power consumption reduction and fastestcalibration computation. As such, one or more of the methodologiesdescribed herein may obviate a need for certain efforts or computingresources. Examples of such computing resources include Processorcycles, network traffic, memory usage, data storage capacity, powerconsumption, network bandwidth, and cooling capacity.

FIG. 1 is a network diagram illustrating an environment 100 suitable foroperating an AR/VR display device 106, according to some exampleembodiments. The environment 100 includes a user 102, an AR/VR displaydevice 106, and a physical object 104. A user 102 operates the AR/VRdisplay device 106. The user 102 may be a human user (e.g., a humanbeing), a machine user (e.g., a computer configured by a softwareprogram to interact with the AR/VR display device 106), or any suitablecombination thereof (e.g., a human assisted by a machine or a machinesupervised by a human). The user 102 is associated with the AR/VRdisplay device 106.

The AR/VR display device 106 may be a computing device with a displaysuch as a smartphone, a tablet computer, or a wearable computing device(e.g., watch or glasses). The computing device may be hand-held or maybe removable mounted to a head of the user 102. In one example, thedisplay includes a screen that displays images captured with a camera ofthe AR/VR display device 106. In another example, the display of thedevice may be transparent such as in lenses of wearable computingglasses. In other examples, the display may be non-transparent,partially transparent, partially opaque. In yet other examples, thedisplay may be wearable by the user 102 to cover the field of vision ofthe user 102.

The AR/VR display device 106 includes an AR application generatesvirtual content based on images detected with the camera of the AR/VRdisplay device 106. For example, the user 102 may point a camera of theAR/VR display device 106 to capture an image of the physical object 104.The AR application generates virtual content corresponding to anidentified object (e.g., physical object 104) in the image and presentsthe virtual content in a display of the AR/VR display device 106.

The AR/VR display device 106 includes a visual inertial tracking system108. The visual inertial tracking system 108 tracks the pose (e.g.,position and orientation) of the AR/VR display device 106 relative tothe real-world environment 110 using, for example, optical sensors(e.g., depth-enabled 3D camera, image camera), inertia sensors (e.g.,gyroscope, accelerometer), wireless sensors (Bluetooth, Wi-Fi), GPSsensor, and audio sensor. In one example, the AR/VR display device 106displays virtual content based on the pose of the AR/VR display device106 relative to the real-world environment 110 and/or the physicalobject 104.

Any of the machines, databases, or devices shown in FIG. 1 may beimplemented in a general-purpose computer modified (e.g., configured orprogrammed) by software to be a special-purpose computer to perform oneor more of the functions described herein for that machine, database, ordevice. For example, a computer system able to implement any one or moreof the methodologies described herein is discussed below with respect toFIG. 7 to FIG. 10 . As used herein, a “database” is a data storageresource and may store data structured as a text file, a table, aspreadsheet, a relational database (e.g., an object-relationaldatabase), a triple store, a hierarchical data store, or any suitablecombination thereof. Moreover, any two or more of the machines,databases, or devices illustrated in FIG. 1 may be combined into asingle machine, and the functions described herein for any singlemachine, database, or device may be subdivided among multiple machines,databases, or devices.

The AR/VR display device 106 may operate over a computer network. Thecomputer network may be any network that enables communication betweenor among machines, databases, and devices. Accordingly, the computernetwork may be a wired network, a wireless network (e.g., a mobile orcellular network), or any suitable combination thereof. The computernetwork may include one or more portions that constitute a privatenetwork, a public network (e.g., the Internet), or any suitablecombination thereof.

FIG. 2 is a block diagram illustrating modules (e.g., components) of theAR/VR display device 106, according to some example embodiments. TheAR/VR display device 106 includes sensors 202, a display 204, aprocessor 208, and a storage device 206. Examples of AR/VR displaydevice 106 include a wearable computing device, a mobile computingdevice, a navigational device, a portable media device, or a smartphone.

The sensors 202 include, for example, an optical sensor 212 (e.g.,camera such as a color camera, a thermal camera, a depth sensor and oneor multiple grayscale, global/rolling shutter tracking cameras) and aninertial sensor 214 (e.g., gyroscope, accelerometer). Other examples ofsensors 202 include a proximity or location sensor (e.g., near fieldcommunication, GPS, Bluetooth, Wi-Fi), an audio sensor (e.g., amicrophone), or any suitable combination thereof. It is noted that thesensors 202 described herein are for illustration purposes and thesensors 202 are thus not limited to the ones described above.

The display 204 includes a screen or monitor configured to displayimages generated by the processor 208. In one example embodiment, thedisplay 204 may be transparent or semi-opaque so that the user 102 cansee through the display 204 (in AR use case). In another exampleembodiment, the display 204 covers the eyes of the user 102 and blocksout the entire field of view of the user 102 (in VR use case). Inanother example, the display 204 includes a touchscreen displayconfigured to receive a user input via a contact on the touchscreendisplay.

The processor 208 includes an AR/VR application 210, a visual inertialtracking system 108, and a tracking calibration module 216. The AR/VRapplication 210 detects and identifies a physical environment or thephysical object 104 using computer vision. The AR/VR application 210retrieves virtual content (e.g., 3D object model) based on theidentified physical object 104 or physical environment. The AR/VRapplication 210 renders the virtual object in the display 204. In oneexample embodiment, the AR/VR application 210 includes a local renderingengine that generates a visualization of virtual content overlaid (e.g.,superimposed upon, or otherwise displayed in tandem with) on an image ofthe physical object 104 captured by the optical sensor 212. Avisualization of the virtual content may be manipulated by adjusting aposition of the physical object 104 (e.g., its physical location,orientation, or both) relative to the optical sensor 212. Similarly, thevisualization of the virtual content may be manipulated by adjusting apose of the AR/VR display device 106 relative to the physical object104. For a VR application, the AR/VR application 210 displays thevirtual content in the display 204 at a location (in the display 204)determined based on a pose of the AR/VR display device 106.

The visual inertial tracking system 108 estimates a pose of the AR/VRdisplay device 106. For example, the visual inertial tracking system 108uses image data and corresponding inertial data from the optical sensor212 and the inertial sensor 214 to track a location and pose of theAR/VR display device 106 relative to a frame of reference (e.g., realworld environment 110). In one example embodiment, the visual inertialtracking system 108 operates independently and asynchronously from theAR/VR application 210. For example, the visual inertial tracking system108 operates offline without receiving any tracking request from theAR/VR application 210. In another example, the visual inertial trackingsystem 108 operates periodically (e.g., every n second, m minutes)regardless of whether the AR/VR application 210 is running at the AR/VRdisplay device 106.

The tracking calibration module 216 initially calibrates the internalvisual odometry system (e.g., optical sensor 212, inertial sensor 214)of the visual inertial tracking system 108 based on default calibrationparameter values (e.g., factory calibration). When the AR/VR application210 operates, the visual inertial tracking system 108 may be referred toas online. When the AR/VR application 210 stops operating, the visualinertial tracking system 108 may be referred to as offline.

In one example embodiment, instead of running the visual inertialtracking system 108 whenever requested by the AR/VR application 210, thetracking calibration module 216 directs the visual inertial trackingsystem 108 to periodically start and run for a short period of timeuntil a new parameter estimation is obtained. The tracking calibrationmodule 216 stores the new estimation in the storage device 206. Thetracking calibration module 216 re-uses the new estimation as a moreup-to-date starting point for another parameter estimation when thevisual inertial tracking system 108 is started the next time. Using thenew estimation as a starting point for another parameter estimationleads to a reduced convergence time and higher tracking accuracyimmediately after the start of the visual inertial tracking system 108.

The storage device 206 stores virtual content 218 and saved onlineestimated parameters 220. The virtual content 218 includes, for example,a database of visual references (e.g., images of physical objects) andcorresponding experiences (e.g., three-dimensional virtual objectmodels). The saved online estimated parameters 220 include, for example,the latest estimated parameter values for the visual inertial trackingsystem 108. In one example, the saved online estimated parameters 220updates the latest estimated parameter value based on the latestestimated parameter value determined by a periodic operation of thevisual inertial tracking system 108.

Any one or more of the modules described herein may be implemented usinghardware (e.g., a Processor of a machine) or a combination of hardwareand software. For example, any module described herein may configure aProcessor to perform the operations described herein for that module.Moreover, any two or more of these modules may be combined into a singlemodule, and the functions described herein for a single module may besubdivided among multiple modules. Furthermore, according to variousexample embodiments, modules described herein as being implementedwithin a single machine, database, or device may be distributed acrossmultiple machines, databases, or devices.

FIG. 3 illustrates the visual inertial tracking system 108 in accordancewith one example embodiment. The visual inertial tracking system 108includes, for example, an inertial sensor module 302, an optical sensormodule 304, and a pose estimation module 306. The inertial sensor module302 accesses inertial sensor data from the inertial sensor 214. Theoptical sensor module 304 accesses optical sensor data from the opticalsensor 212.

The pose estimation module 306 determines a pose (e.g., location,position, orientation) of the AR/VR display device 106 relative to aframe of reference (e.g., real world environment 110). In one exampleembodiment, the pose estimation module 306 includes a visual odometrysystem that estimates the pose of the AR/VR display device 106 based on3D maps of feature points from images captured with the optical sensor212 and the inertial sensor data captured with the inertial sensor 214.The optical sensor module 304 accesses image data from the opticalsensor 212.

In one example embodiment, the pose estimation module 306 computes theposition and orientation of the AR/VR display device 106. The AR/VRdisplay device 106 includes one or more optical sensor 212 mounted on arigid platform (a frame of the AR/VR display device 106) with one ormore inertial sensor 214. The optical sensor 212 can be mounted withnon-overlapping (distributed aperture) or overlapping (stereo or more)fields-of-view.

In some example embodiments, the pose estimation module 306 includes analgorithm that combines inertial information from the inertial sensor214 and image information from the optical sensor 212 that are coupledto a rigid platform (e.g., AR/VR display device 106) or a rig. In oneembodiment, a rig may consist of multiple cameras mounted on a rigidplatform with an inertial navigation unit (e.g., inertial sensor 214). Arig may thus have at least one inertial navigation unit and at least onecamera.

FIG. 4 is a block diagram illustrating a tracking calibration module 216in accordance with one example embodiment. The tracking calibrationmodule 216 includes an online parameter estimation component 402 and aperiodic parameter estimation component 404.

The online parameter estimation component 402 operates the visualinertial tracking system 108 in response to detecting that the AR/VRapplication 210 is requesting a tracking operation. In one exampleembodiment, the online parameter estimation component 402 accesses alatest estimated calibration parameter values stored in the saved onlineestimated parameters 220. The online parameter estimation component 402uses the latest estimated parameter values to calculate updatedcalibration parameter values.

The periodic parameter estimation component 404 periodically operatesthe visual inertial tracking system 108 to calculate the latestcalibration parameter values. In one example, the periodic parameterestimation component 404 periodically operates the visual inertialtracking system 108 every n seconds. In another example, the periodicparameter estimation component 404 operates the visual inertial trackingsystem 108 in response to detecting a trigger event (e.g., temperaturesensor indicates large change, accelerometer detects large change (e.g.,device falling on the floor), “wear”-detector triggers (e.g., userputting on/wearing AR glasses, which causes mechanical stress on theframe), low battery power). The periodic parameter estimation component404 stops the visual inertial tracking system 108 after the latestup-to-date estimated parameter value is available. The periodicparameter estimation component 404 stores the up-to-date estimatedparameter value as the latest saved online estimated parameters 220 instorage device 206.

FIG. 5 is a flow diagram illustrating a parameter estimation operation516 in accordance with one example embodiment. Operations in theparameter estimation operation 516 may be performed by the visualinertial tracking system 108, using Components (e.g., modules, engines)described above with respect to FIG. 2 and FIG. 3 . Accordingly, theparameter estimation operation 516 is described by way of example withreference to the visual inertial tracking system 108 and trackingcalibration module 216. However, it shall be appreciated that at leastsome of the operations may be deployed on various other hardwareconfigurations or be performed by similar Components residing elsewhere.For example, some of the operations may be performed at the AR/VRapplication 210.

At decision block 502, the tracking calibration module 216 determineswhether the AR/VR application 210 is requesting tracking operations fromthe visual inertial tracking system 108. If the tracking calibrationmodule 216 determines that the AR/VR application 210 is requestingtracking operations from the visual inertial tracking system 108, theprocess proceeds to block 510, block 512, and decision block 514.

At block 510, the tracking calibration module 216 loads or retrieves alatest estimated parameter value from the storage device 206. At block512, the tracking calibration module 216 calculates an updated latestcalibration parameter value starting from the retrieved estimatedparameter value. At decision block 514, the tracking calibration module216 determines whether the AR/VR application 210 still requires trackingoperations from the visual inertial tracking system 108. If the AR/VRapplication 210 does not need any further tracking operations from thevisual inertial tracking system 108, the process returns back todecision block 502.

If the tracking calibration module 216 determines that the AR/VRapplication 210 is not requesting tracking operations from the visualinertial tracking system 108, the process continues to decision block504, block 506, and block 508. At decision block 504, the trackingcalibration module 216 determines whether the re-estimate the latestparameter value based on a last operation of the AR/VR application 210.At block 506, the tracking calibration module 216 operates the visualinertial tracking system 108 offline (e.g., without running the AR/VRapplication 210). At block 508, the tracking calibration module 216saves the latest estimated parameter value and stops a trackingoperation of the visual inertial tracking system 108.

FIG. 6 is a block diagram illustrating an example process in accordancewith one example embodiment. The visual inertial tracking system 108receives sensor data from sensors 202 to determine a pose of the AR/VRdisplay device 106. The tracking calibration module 216 calibrates thesensor data based on the latest parameter estimated values (e.g.,spatial relations between the cameras and IMU, IMU biases, bending ofthe frame resulting a displacement of sensors disposed at pre-definedlocations in the AR/VR display device 106, auto exposure). The trackingcalibration module 216 stores the latest estimated parameter values(e.g., saved online estimated parameters 220) in the storage device 206.

The calibrated spatial relations data is provided to a model 602 of thevisual inertial tracking system 108. The model 602 is initiallydetermined with the factory calibration 604 and the data from the visualinertial tracking system 108. In one example embodiment, the model 602is determined based on data from the visual inertial tracking system 108and the saved online estimated parameters 220. The model 602 providesthe geometric model of the AR/VR display device 106 to the AR/VRapplication 210.

The AR/VR application 210 retrieves virtual content 218 from the storagedevice 206 and causes the virtual content 218 to be displayed at alocation based on the geometric model of the AR/VR display device 106.

FIG. 7 is a flow diagram illustrating a routine 700 for updating visualodometry of an AR display device in accordance with one exampleembodiment. Operations in the routine 700 may be performed by the visualinertial tracking system 108, using Components (e.g., modules, engines)described above with respect to FIG. 2 . Accordingly, the routine 700 isdescribed by way of example with reference to the tracking calibrationmodule 216. However, it shall be appreciated that at least some of theoperations of the routine 700 may be deployed on various other hardwareconfigurations or be performed by similar Components residing elsewhere.

In block 702, the periodic parameter estimation component 404periodically operates the visual inertial tracking system 108. In block704, the visual inertial tracking system 108 determines the latestestimated parameter value. In block 706, the periodic parameterestimation component 404 stores the latest estimated parameter value.

It is to be noted that other embodiments may use different sequencing,additional or fewer operations, and different nomenclature orterminology to accomplish similar functions. In some embodiments,various operations may be performed in parallel with other operations,either in a synchronous or asynchronous manner. The operations describedherein were chosen to illustrate some principles of operations in asimplified form.

FIG. 8 is a flow diagram illustrating a routine 800 for updating visualodometry of an AR display device in accordance with one exampleembodiment. Operations in the routine 800 may be performed by thetracking calibration module 216, using Components (e.g., modules,engines) described above with respect to FIG. 2 . Accordingly, theroutine 800 is described by way of example with reference to thetracking calibration module 216. However, it shall be appreciated thatat least some of the operations of the routine 800 may be deployed onvarious other hardware configurations or be performed by similarcomponents residing elsewhere.

In block 802, the online parameter estimation component 402 detects anapplication requesting the visual inertial tracking system 108. In block804, the online parameter estimation component 402 retrieves the latestestimated parameter value from the storage device 206. In block 806, theonline parameter estimation component 402 calibrates the visual inertialtracking system 108 based on the latest estimated parameter value andfactory calibration.

FIG. 9 is a flow diagram illustrating a routine 900 for updating visualodometry of an AR display device in accordance with one exampleembodiment. Operations in the routine 900 may be performed by thetracking calibration module 216, using Components (e.g., modules,engines) described above with respect to FIG. 3 . Accordingly, theroutine 900 is described by way of example with reference to thetracking calibration module 216. However, it shall be appreciated thatat least some of the operations of the routine 900 may be deployed onvarious other hardware configurations or be performed by similarComponents residing elsewhere.

In block 902, the periodic parameter estimation component 404 detects atrigger event. In block 904, the periodic parameter estimation component404 starts the visual inertial tracking system 108 in response to thetrigger event. In block 906, the periodic parameter estimation component404 determines a latest estimated parameter value. In block 908, theperiodic parameter estimation component 404 stores the latest estimatedparameter value in the storage device 206. In block 910, the periodicparameter estimation component 404 turns off the visual inertialtracking system 108.

FIG. 10 is a flow diagram illustrating a method for calibrating avisual-inertial tracking system in accordance with one embodiment. Inblock 1002, routine 1000 operates, at a device, the visual-inertialtracking system without receiving a tracking request from a virtualobject display application. In block 1004, routine 1000 in response tooperating the visual-inertial tracks system, accessing sensor data froma plurality of sensors of the device. In block 1006, routine 1000identifies, based on the sensor data, a first calibration parametervalue of the visual-inertial tracking system. In block 1008, routine1000 stores the first calibration parameter value. In block 1010,routine 1000 detects the tracking request from the virtual objectdisplay application to the visual-inertial tracking system. In block1012, routine 1000 in response to detecting the tracking request,accesses the first calibration parameter value and determining a secondcalibration parameter value from the first calibration parameter value.

FIG. 11 is a block diagram 1100 illustrating a software architecture1104, which can be installed on any one or more of the devices describedherein. The software architecture 1104 is supported by hardware such asa machine 1102 that includes Processors 1120, memory 1126, and I/OComponents 1138. In this example, the software architecture 1104 can beconceptualized as a stack of layers, where each layer provides aparticular functionality. The software architecture 1104 includes layerssuch as an operating system 1112, libraries 1110, frameworks 1108, andapplications 1106. Operationally, the applications 1106 invoke API calls1150 through the software stack and receive messages 1152 in response tothe API calls 1150.

The operating system 1112 manages hardware resources and provides commonservices. The operating system 1112 includes, for example, a kernel1114, services 1116, and drivers 1122. The kernel 1114 acts as anabstraction layer between the hardware and the other software layers.For example, the kernel 1114 provides memory management, Processormanagement (e.g., scheduling), Component management, networking, andsecurity settings, among other functionalities. The services 1116 canprovide other common services for the other software layers. The drivers1122 are responsible for controlling or interfacing with the underlyinghardware. For instance, the drivers 1122 can include display drivers,camera drivers, BLUETOOTH® or BLUETOOTH® Low Energy drivers, flashmemory drivers, serial communication drivers (e.g., Universal Serial Bus(USB) drivers), WI-FI® drivers, audio drivers, power management drivers,and so forth.

The libraries 1110 provide a low-level common infrastructure used by theapplications 1106. The libraries 1110 can include system libraries 1118(e.g., C standard library) that provide functions such as memoryallocation functions, string manipulation functions, mathematicfunctions, and the like. In addition, the libraries 1110 can include APIlibraries 1124 such as media libraries (e.g., libraries to supportpresentation and manipulation of various media formats such as MovingPicture Experts Group-4 (MPEG4), Advanced Video Coding (H.264 or AVC),Moving Picture Experts Group Layer-3 (MP3), Advanced Audio Coding (AAC),Adaptive Multi-Rate (AMR) audio codec, Joint Photographic Experts Group(JPEG or JPG), or Portable Network Graphics (PNG)), graphics libraries(e.g., an OpenGL framework used to render in two dimensions (2D) andthree dimensions (3D) in a graphic content on a display), databaselibraries (e.g., SQLite to provide various relational databasefunctions), web libraries (e.g., WebKit to provide web browsingfunctionality), and the like. The libraries 1110 can also include a widevariety of other libraries 1128 to provide many other APIs to theapplications 1106.

The frameworks 1108 provide a high-level common infrastructure that isused by the applications 1106. For example, the frameworks 1108 providevarious graphical user interface (GUI) functions, high-level resourcemanagement, and high-level location services. The frameworks 1108 canprovide a broad spectrum of other APIs that can be used by theapplications 1106, some of which may be specific to a particularoperating system or platform.

In an example embodiment, the applications 1106 may include a homeapplication 1136, a contacts application 1130, a browser application1132, a book reader application 1134, a location application 1142, amedia application 1144, a messaging application 1146, a game application1148, and a broad assortment of other applications such as a third-partyapplication 1140. The applications 1106 are programs that executefunctions defined in the programs. Various programming languages can beemployed to create one or more of the applications 1106, structured in avariety of manners, such as object-oriented programming languages (e.g.,Objective-C, Java, or C++) or procedural programming languages (e.g., Cor assembly language). In a specific example, the third-partyapplication 1140 (e.g., an application developed using the ANDROID™ orIOS™ software development kit (SDK) by an entity other than the vendorof the particular platform) may be mobile software running on a mobileoperating system such as IOS™, ANDROID™, WINDOWS® Phone, or anothermobile operating system. In this example, the third-party application1140 can invoke the API calls 1150 provided by the operating system 1112to facilitate functionality described herein.

FIG. 12 is a diagrammatic representation of the machine 1200 withinwhich instructions 1208 (e.g., software, a program, an application, anapplet, an app, or other executable code) for causing the machine 1200to perform any one or more of the methodologies discussed herein may beexecuted. For example, the instructions 1208 may cause the machine 1200to execute any one or more of the methods described herein. Theinstructions 1208 transform the general, non-programmed machine 1200into a particular machine 1200 programmed to carry out the described andillustrated functions in the manner described. The machine 1200 mayoperate as a standalone device or may be coupled (e.g., networked) toother machines. In a networked deployment, the machine 1200 may operatein the capacity of a server machine or a client machine in aserver-client network environment, or as a peer machine in apeer-to-peer (or distributed) network environment. The machine 1200 maycomprise, but not be limited to, a server computer, a client computer, apersonal computer (PC), a tablet computer, a laptop computer, a netbook,a set-top box (STB), a PDA, an entertainment media system, a cellulartelephone, a smart phone, a mobile device, a wearable device (e.g., asmart watch), a smart home device (e.g., a smart appliance), other smartdevices, a web appliance, a network router, a network switch, a networkbridge, or any machine capable of executing the instructions 1208,sequentially or otherwise, that specify actions to be taken by themachine 1200. Further, while only a single machine 1200 is illustrated,the term “machine” shall also be taken to include a collection ofmachines that individually or jointly execute the instructions 1208 toperform any one or more of the methodologies discussed herein.

The machine 1200 may include Processors 1202, memory 1204, and I/OComponents 1242, which may be configured to communicate with each othervia a bus 1244. In an example embodiment, the Processors 1202 (e.g., aCentral Processing Unit (CPU), a Reduced Instruction Set Computing(RISC) Processor, a Complex Instruction Set Computing (CISC) Processor,a Graphics Processing Unit (GPU), a Digital Signal Processor (DSP), anASIC, a Radio-Frequency Integrated Circuit (RFIC), another Processor, orany suitable combination thereof) may include, for example, a Processor1206 and a Processor 1210 that execute the instructions 1208. The term“Processor” is intended to include multi-core Processors that maycomprise two or more independent Processors (sometimes referred to as“cores”) that may execute instructions contemporaneously. Although FIG.12 shows multiple Processors 1202, the machine 1200 may include a singleProcessor with a single core, a single Processor with multiple cores(e.g., a multi-core Processor), multiple Processors with a single core,multiple Processors with multiples cores, or any combination thereof.

The memory 1204 includes a main memory 1212, a static memory 1214, and astorage unit 1216, both accessible to the Processors 1202 via the bus1244. The main memory 1204, the static memory 1214, and storage unit1216 store the instructions 1208 embodying any one or more of themethodologies or functions described herein. The instructions 1208 mayalso reside, completely or partially, within the main memory 1212,within the static memory 1214, within machine-readable medium 1218within the storage unit 1216, within at least one of the Processors 1202(e.g., within the Processor's cache memory), or any suitable combinationthereof, during execution thereof by the machine 1200.

The I/O Components 1242 may include a wide variety of Components toreceive input, provide output, produce output, transmit information,exchange information, capture measurements, and so on. The specific I/OComponents 1242 that are included in a particular machine will depend onthe type of machine. For example, portable machines such as mobilephones may include a touch input device or other such input mechanisms,while a headless server machine will likely not include such a touchinput device. It will be appreciated that the I/O Components 1242 mayinclude many other Components that are not shown in FIG. 12 . In variousexample embodiments, the I/O Components 1242 may include outputComponents 1228 and input Components 1230. The output Components 1228may include visual Components (e.g., a display such as a plasma displaypanel (PDP), a light emitting diode (LED) display, a liquid crystaldisplay (LCD), a projector, or a cathode ray tube (CRT)), acousticComponents (e.g., speakers), haptic Components (e.g., a vibratory motor,resistance mechanisms), other signal generators, and so forth. The inputComponents 1230 may include alphanumeric input Components (e.g., akeyboard, a touch screen configured to receive alphanumeric input, aphoto-optical keyboard, or other alphanumeric input Components),point-based input Components (e.g., a mouse, a touchpad, a trackball, ajoystick, a motion sensor, or another pointing instrument), tactileinput Components (e.g., a physical button, a touch screen that provideslocation and/or force of touches or touch gestures, or other tactileinput Components), audio input Components (e.g., a microphone), and thelike.

In further example embodiments, the I/O Components 1242 may includebiometric Components 1232, motion Components 1234, environmentalComponents 1236, or position Components 1238, among a wide array ofother Components. For example, the biometric Components 1232 includeComponents to detect expressions (e.g., hand expressions, facialexpressions, vocal expressions, body gestures, or eye tracking), measurebiosignals (e.g., blood pressure, heart rate, body temperature,perspiration, or brain waves), identify a person (e.g., voiceidentification, retinal identification, facial identification,fingerprint identification, or electroencephalogram-basedidentification), and the like. The motion Components 1234 includeacceleration sensor Components (e.g., accelerometer), gravitation sensorComponents, rotation sensor Components (e.g., gyroscope), and so forth.The environmental Components 1236 include, for example, illuminationsensor Components (e.g., photometer), temperature sensor Components(e.g., one or more thermometers that detect ambient temperature),humidity sensor Components, pressure sensor Components (e.g.,barometer), acoustic sensor Components (e.g., one or more microphonesthat detect background noise), proximity sensor Components (e.g.,infrared sensors that detect nearby objects), gas sensors (e.g., gasdetection sensors to detection concentrations of hazardous gases forsafety or to measure pollutants in the atmosphere), or other Componentsthat may provide indications, measurements, or signals corresponding toa surrounding physical environment. The position Components 1238 includelocation sensor Components (e.g., a GPS receiver Component), altitudesensor Components (e.g., altimeters or barometers that detect airpressure from which altitude may be derived), orientation sensorComponents (e.g., magnetometers), and the like.

Communication may be implemented using a wide variety of technologies.The I/O Components 1242 further include communication Components 1240operable to couple the machine 1200 to a network 1220 or devices 1222via a coupling 1224 and a coupling 1226, respectively. For example, thecommunication Components 1240 may include a network interface Componentor another suitable device to interface with the network 1220. Infurther examples, the communication Components 1240 may include wiredcommunication Components, wireless communication Components, cellularcommunication Components, Near Field Communication (NFC) Components,Bluetooth® Components (e.g., Bluetooth® Low Energy), WiFi® Components,and other communication Components to provide communication via othermodalities. The devices 1222 may be another machine or any of a widevariety of peripheral devices (e.g., a peripheral device coupled via aUSB).

Moreover, the communication Components 1240 may detect identifiers orinclude Components operable to detect identifiers. For example, thecommunication Components 1240 may include Radio Frequency Identification(RFID) tag reader Components, NFC smart tag detection Components,optical reader Components (e.g., an optical sensor to detectone-dimensional bar codes such as Universal Product Code (UPC) bar code,multi-dimensional bar codes such as Quick Response (QR) code, Azteccode, Data Matrix, Dataglyph, MaxiCode, PDF417, Ultra Code, UCC RSS-2Dbar code, and other optical codes), or acoustic detection Components(e.g., microphones to identify tagged audio signals). In addition, avariety of information may be derived via the communication Components1240, such as location via Internet Protocol (IP) geolocation, locationvia Wi-Fi® signal triangulation, location via detecting an NFC beaconsignal that may indicate a particular location, and so forth.

The various memories (e.g., memory 1204, main memory 1212, static memory1214, and/or memory of the Processors 1202) and/or storage unit 1216 maystore one or more sets of instructions and data structures (e.g.,software) embodying or used by any one or more of the methodologies orfunctions described herein. These instructions (e.g., the instructions1208), when executed by Processors 1202, cause various operations toimplement the disclosed embodiments.

The instructions 1208 may be transmitted or received over the network1220, using a transmission medium, via a network interface device (e.g.,a network interface Component included in the communication Components1240) and using any one of a number of well-known transfer protocols(e.g., hypertext transfer protocol (HTTP)). Similarly, the instructions1208 may be transmitted or received using a transmission medium via thecoupling 1226 (e.g., a peer-to-peer coupling) to the devices 1222.

Although an embodiment has been described with reference to specificexample embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader scope of the present disclosure. Accordingly, the specificationand drawings are to be regarded in an illustrative rather than arestrictive sense. The accompanying drawings that form a part hereof,show by way of illustration, and not of limitation, specific embodimentsin which the subject matter may be practiced. The embodimentsillustrated are described in sufficient detail to enable those skilledin the art to practice the teachings disclosed herein. Other embodimentsmay be utilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in a single embodiment for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus, the following claimsare hereby incorporated into the Detailed Description, with each claimstanding on its own as a separate embodiment.

Examples

Example 1 is a method for calibrating a visual-inertial tracking systemcomprising: operating, at a device, the visual-inertial tracking systemwithout receiving a tracking request from a virtual object displayapplication; in response to operating the visual-inertial trackingsystem, accessing sensor data from a plurality of sensors of the device;identifying, based on the sensor data, a first calibration parametervalue of the visual-inertial tracking system; storing the firstcalibration parameter value; detecting the tracking request from thevirtual object display application to the visual-inertial trackingsystem; and in response to detecting the tracking request, accessing thefirst calibration parameter value and determining a second calibrationparameter value from the first calibration parameter value.

Example 2 includes example 1, wherein operating the visual-inertialtracking system further comprises: periodically accessing the sensordata from the plurality of sensors, wherein the visual-inertial trackingsystem operates independently from the virtual object displayapplication.

Example 3 includes example 1, further comprising: detecting acalibration trigger event at the device, wherein operating thevisual-inertial tracking system is in response to detecting thecalibration trigger event.

Example 4 includes example 3, wherein the trigger event comprises atleast one of a temperature change exceeding a temperature threshold, anaccelerometer sensor value exceeding an accelerometer threshold value, abattery level of the device exceeding a battery threshold value, auser-activation of the device, or a detection that the device is worn bya user.

Example 5 includes example 1, wherein identifying the first calibrationparameter value is based on a convergence between a first virtual objectdata point location and a second virtual object data point location, thefirst virtual object data point location being determined based on thesensor data that are adjusted with the first parameter value, the secondvirtual object data point location being determined based on the sensordata that are adjusted with a default calibration parameter value of thedevice.

Example 6 includes example 1, further comprising: turning off thevisual-inertial tracking system after storing the first calibrationparameter value.

Example 7 includes example 1, further comprising: calibrating thevisual-inertial tracking system with the first calibration parametervalue before detecting the tracking request from the virtual objectdisplay application.

Example 8 includes example 1, further comprising: calibrating thevisual-inertial tracking system with the second calibration parametervalue after detecting the tracking request from the virtual objectdisplay application.

Example 9 includes example 1, wherein determining the second calibrationparameter value further comprises: replacing a starting value comprisingthe default calibration value with the first calibration parametervalue; and performing a calibration of the visual-inertial trackingsystem starting with the starting value.

Example 10 includes example 1, wherein storing the first calibrationparameter value further comprises: storing the first calibration valuein a storage device of the device or at a server.

Example 11 is a computing apparatus comprising: a processor; and amemory storing instructions that, when executed by the processor,configure the apparatus to perform operations comprising: operate, at adevice, the visual-inertial tracking system without receiving a trackingrequest from a virtual object display application; in response tooperating the visual-inertial track system, accessing sensor data from aplurality of sensors of the device; identify, based on the sensor data,a first calibration parameter value of the visual-inertial trackingsystem; store the first calibration parameter value; detect the trackingrequest from the virtual object display application to thevisual-inertial tracking system; and in response to detecting thetracking request, access the first calibration parameter value anddetermining a second calibration parameter value from the firstcalibration parameter value.

Example 12 includes example 11, wherein operating the visual-inertialtrack system further comprises: periodically access the sensor data fromthe plurality of sensors, wherein the visual-inertial track systemoperates independently from the virtual object display application.

Example 13 includes example 11, wherein the instructions furtherconfigure the apparatus to: detect a calibration trigger event at thedevice, wherein operating the visual-inertial track system is inresponse to detecting the calibration trigger event.

Example 14 includes example 13, wherein the trigger event comprises atleast one of a temperature change exceeding a temperature threshold, anaccelerometer sensor value exceeding an accelerometer threshold value, abattery level of the device exceeding a battery threshold value, auser-activation of the device, or a detection that the device is worn bya user.

Example 15 includes example 11, wherein identifying the firstcalibration parameter value is based on a convergence between a firstvirtual object data point location and a second virtual object datapoint location, the first virtual object data point location beingdetermined based on the sensor data that are adjusted with the firstparameter value, the second virtual object data point location beingdetermined based on the sensor data that are adjusted with a defaultcalibration parameter value of the device.

Example 16 includes example 11, wherein the instructions furtherconfigure the apparatus to: turn off the visual-inertial tracking systemafter storing the first calibration parameter value.

Example 17 includes example 11, wherein the instructions furtherconfigure the apparatus to: calibrate the visual-inertial trackingsystem with the first calibration parameter value before detecting thetracking request from the virtual object display application.

Example 18 includes example 11, wherein the instructions furtherconfigure the apparatus to: calibrate the visual-inertial trackingsystem with the second calibration parameter value after detecting thetracking request from the virtual object display application.

Example 19 includes example 11, wherein determining the secondcalibration parameter value further comprises: replace a starting valuecomprising the default calibration value with the first calibrationparameter value; and perform a calibration of the visual-inertialtracking system starting with the starting value.

Example 20 includes a non-transitory computer-readable storage medium,the computer-readable storage medium including instructions that whenexecuted by a computer, cause the computer to perform operationscomprising: operate, at a device, a visual-inertial tracking systemwithout receiving a tracking request from a virtual object displayapplication; in response to operating the visual-inertial track system,accessing sensor data from a plurality of sensors of the device;identify, based on the sensor data, a first calibration parameter valueof the visual-inertial tracking system; store the first calibrationparameter value; detect the tracking request from the virtual objectdisplay application to the visual-inertial tracking system; and inresponse to detecting the tracking request, access the first calibrationparameter value and determining a second calibration parameter valuefrom the first calibration parameter value.

What is claimed is:
 1. A method for calibrating a visual-inertialtracking system comprising: detecting, at a head-mounted device, that avirtual object display application that is configured to operate at thehead-mounted device is not requesting tracking operations from thevisual-inertial tracking system of the head-mounted device; in responseto the detection, operating, at the head-mounted device, thevisual-inertial tracking system by accessing sensor data from aplurality of sensors only at the head-mounted device; identifying, atthe head-mounted device, a first calibration parameter value of thevisual-inertial tracking system based on the sensor data from theplurality of sensors only at the head-mounted device; storing, in astorage of the head-mounted device, the first calibration parametervalue; detecting an operation of the virtual object display applicationat the head-mounted device by detecting a tracking request from thevirtual object display application to the visual-inertial trackingsystem; and in response to detecting the tracking request, accessing, atthe storage of the head-mounted device, the first calibration parametervalue and determining, at the head-mounted device, a second calibrationparameter value of the virtual-inertial tracking system from the firstcalibration parameter value.
 2. The method of claim 1, wherein operatingthe visual-inertial tracking system further comprises: periodicallyaccessing the sensor data from the plurality of sensors, wherein thevisual-inertial tracking system operates independently from the virtualobject display application.
 3. The method of claim 1, furthercomprising: detecting a calibration trigger event at the head-mounteddevice, wherein operating the visual-inertial tracking system is inresponse to detecting the calibration trigger event.
 4. The method ofclaim 3, wherein the trigger event comprises at least one of atemperature change exceeding a temperature threshold, an accelerometersensor value exceeding an accelerometer threshold value, a battery levelof the device exceeding a battery threshold value, a user-activation ofthe head-mounted device, or a detection that the head-mounted device isworn by a user.
 5. The method of claim 1, wherein identifying the firstcalibration parameter value is based on a convergence between a firstvirtual object data point location and a second virtual object datapoint location, the first virtual object data point location beingdetermined based on the sensor data that are adjusted with the firstparameter value, the second virtual object data point location beingdetermined based on the sensor data that are adjusted with a defaultcalibration parameter value of the head-mounted device.
 6. The method ofclaim 1, further comprising: turning off the visual-inertial trackingsystem after storing the first calibration parameter value; andactivating the visual-inertial tracking system in response to detectingan operation of the virtual object display application.
 7. The method ofclaim 1, further comprising: calibrating the visual-inertial trackingsystem with the first calibration parameter value before operating thevirtual object display application.
 8. The method of claim 1, furthercomprising: calibrating the visual-inertial tracking system with thesecond calibration parameter value after detecting the tracking requestfrom the virtual object display application.
 9. The method of claim 1,wherein determining the second calibration parameter value furthercomprises: replacing a starting value comprising a default calibrationvalue with the first calibration parameter value; and performing acalibration of the visual-inertial tracking system starting with thestarting value.
 10. The method of claim 1, wherein storing the firstcalibration parameter value further comprises: storing the firstcalibration value in at a server.
 11. A computing apparatus comprising:a processor; and a memory storing instructions that, when executed bythe processor, configure the apparatus to perform operations comprising:detect, at a head-mounted device, that a virtual object displayapplication that is configured to operate at the head-mounted device isnot requesting tracking operations from the visual-inertial trackingsystem of the head-mounted device; in response to the detection,operate, at the head-mounted device, the visual-inertial tracking systemby accessing sensor data from a plurality of sensors only at thehead-mounted device; identify, at the head-mounted device, a firstcalibration parameter value of the visual-inertial tracking system basedon the sensor data from the plurality of sensors only at thehead-mounted device; store, in a storage of the head-mounted device, thefirst calibration parameter value; detect an operation of the virtualobject display application at the head-mounted device by detecting atracking request from the virtual object display application to thevisual-inertial tracking system; and in response to detecting thetracking request, access, at the storage of the head-mounted device, thefirst calibration parameter value and determining, at the head-mounteddevice, a second calibration parameter value of the virtual-inertialtracking system from the first calibration parameter value.
 12. Thecomputing apparatus of claim 11, wherein operating the visual-inertialtrack system further comprises: periodically access the sensor data fromthe plurality of sensors, wherein the visual-inertial track systemoperates independently from the virtual object display application. 13.The computing apparatus of claim 11, wherein the instructions furtherconfigure the apparatus to: detect a calibration trigger event at thehead-mounted device, wherein operating the visual-inertial track systemis in response to detecting the calibration trigger event.
 14. Thecomputing apparatus of claim 13, wherein the trigger event comprises atleast one of a temperature change exceeding a temperature threshold, anaccelerometer sensor value exceeding an accelerometer threshold value, abattery level of the head-mounted device exceeding a battery thresholdvalue, a user-activation of the head-mounted device, or a detection thatthe head-mounted device is worn by a user.
 15. The computing apparatusof claim 11, wherein identifying the first calibration parameter valueis based on a convergence between a first virtual object data pointlocation and a second virtual object data point location, the firstvirtual object data point location being determined based on the sensordata that are adjusted with the first parameter value, the secondvirtual object data point location being determined based on the sensordata that are adjusted with a default calibration parameter value of thehead-mounted device.
 16. The computing apparatus of claim 11, whereinthe instructions further configure the apparatus to: turn off thevisual-inertial tracking system after storing the first calibrationparameter value; and activating the visual-inertial tracking system inresponse to detecting an operation of the virtual object displayapplication.
 17. The computing apparatus of claim 11, wherein theinstructions further configure the apparatus to: calibrate thevisual-inertial tracking system with the first calibration parametervalue before operating the virtual object display application.
 18. Thecomputing apparatus of claim 11, wherein the instructions furtherconfigure the apparatus to: calibrate the visual-inertial trackingsystem with the second calibration parameter value after detecting thetracking request from the virtual object display application.
 19. Thecomputing apparatus of claim 11, wherein determining the secondcalibration parameter value further comprises: replace a starting valuecomprising a default calibration value with the first calibrationparameter value; and perform a calibration of the visual-inertialtracking system starting with the starting value.
 20. A non-transitorycomputer-readable storage medium, the computer-readable storage mediumincluding instructions that when executed by a computer, cause thecomputer to perform operations comprising: detect, at a head-mounteddevice, that a virtual object display application that is configured tooperate at the head-mounted device is not requesting tracking operationsfrom the visual-inertial tracking system of the head-mounted device; inresponse to the detection, operate, at the head-mounted device, thevisual-inertial tracking system by accessing sensor data from aplurality of sensors only at the head-mounted device; identify, at thehead-mounted device, a first calibration parameter value of thevisual-inertial tracking system based on the sensor data from theplurality of sensors only at the head-mounted device; store, in astorage of the head-mounted device, the first calibration parametervalue; detect an operation of the virtual object display application atthe head-mounted device by detecting a tracking request from the virtualobject display application to the visual-inertial tracking system; andin response to detecting the tracking request, access, at the storage ofthe head-mounted device, the first calibration parameter value anddetermining, at the head-mounted device, a second calibration parametervalue of the virtual-inertial tracking system from the first calibrationparameter value.